1. Field of the Invention
The present invention relates to a method for corner contouring of flat glass substrates, such as, for example, for household appliances, in a continuous feed-through process.
2. Description of Related Art
Edge grinding represents an important part of forming post-processing of flat glass substrates. This process step is generally performed in a continuous feed-through method, in which, first of all, the lengthwise edges are simultaneously worked, after which the flat glass substrate is rotated by 90° and then the transverse edges are ground to final dimension. In this process, feed speeds of up to 12 m/min can be achieved at present. For large end dimensions, such as those occurring in the field of household appliances, for example, unit quantities of up to 1000 units/hour can be achieved based on these feed speeds.
At such high feed speeds, however, a specific working of the corners of flat glass substrates is generally dispensed with or else they are only broken off by a simple, mechanically controlled, and often inaccurate edge impact.
Thus, for example, EP 1 190 816 B1, discloses a corner grinding unit for working the corners of flat glass plates in a continuous feed-through process in which a grinding tool is positioned and guided under force within a cage constructed from guide rollers in such a way that the worked corner is broken off in the form of a 45° bevel.
Also disclosed in JP 20011239443 A are a method and an apparatus in which a glass substrate is advanced against a contouring tool in a continuous feed-through movement. The contouring tools are movably positioned solely perpendicular to the feed direction of the glass substrate and are moved synchronously to the continuous feed-through movement in such a way that the corners, in turn, are broken off in the form of a 45° bevel.
A drawback in each case is, on the one hand, a low precision of corner working and, on the other hand, a limited freedom of design for the corners.
A somewhat different approach is disclosed in DE 3035612, for example, which permits rounding of the corners of glass plates in a continuous feed-through process. The device has a periphery grinding wheel, which is suspended on a pivotable and spring-loaded arm. Although the rounding of corners affords a greater diversity of shapes, the reproducibility remains very limited, however, owing to inadequate axial control.
The same holds true also for an apparatus that has already been commercially marketed, in which a tool is guided on a carriage, positioned at a 45° angle, with modulated speed in such a manner that a radial geometry, albeit imprecise, is created at the individual corners.
With increasing demands for constantly higher accuracies and new esthetic characteristics due to more flexible or asymmetric or generally more complex contours, for example, in particular in the case of glasses for household appliances, such as fireplace glass, refrigerator components, control panels, or cooktops, manufacturers are being compelled to resort to slower and significantly more expensive processing methods involving the use of stationary grinding machines. A high precision is achieved hereby at the transitions between the lengthwise or transverse edges and the corners by operation in a single complete cycle without setting down or even exchanging the tool. However, the use of such stationary grinding machines allows throughputs of only approximately 100 units/hour per tool spindle employed to be achieved.
It would be desirable to unite the precision and variability that can be achieved with the stationary technique with the economy of the continuous feed-through process. In order to come closer to this goal, there have been additional approaches in recent years for integrating the corner working into continuous feed-through process equipment.
The most recent developments have been based on modern CNC control techniques and permit the working of complex contours at the corners in a completely continuous feed-through process that still has a throughput of up to 400 units/hours for flat glass substrates having the format of typical glasses for household appliances. The contouring tools can travel in two dimensions and are conveyed synchronously with the continuous feed-through movement of the work piece over a control path. The synchronized movement is triggered beforehand by the work piece passing the work station. This operation is referred to herein as synchronization of the movement of the contouring tool to the continuous feed-through movement of the flat glass substrate. Under programmed control, the tool approaches the corners and then travels over the programmed track. This operation is referred to as contouring, in which a relative movement between the contouring tool and the flat glass substrate is performed so as to overlap the continuous feed-through movement. With these apparatuses it is possible not only to work the corners but also to work the entire lengthwise edge in a complex contour with improved quality. The contouring tool, together with its CNC-controlled drive unit, constitutes a CNC unit.
A device for conducting a contouring method, which has a first table movable in the direction of continuous feed-through movement of the flat glass substrate and a second table, which is disposed on the first table, is movable perpendicular to the continuous feed-through movement, and which bears the contouring tool and its drive unit, is known from the document U.S. Pat. No. 6,887,140 B2. Each table has a stop that can be advanced against the flat glass substrate. The contouring takes place here on a flat glass substrate that is clamped in place.
The document US 2010/0279588 A1 describes a method of this type and the device related to it for corner contouring of flat glass substrates, in which the movement of a passing flat glass substrate is detected by means of a sensor and the movement of the grinding tool in the direction of feed is controlled so that the relative movement between tool and flat glass amounts to a constant zero, inasmuch as this is possible. After this, the grinding of the corners ensues.
However, a high-precision creation of corner radii is also not ensured in these solutions. The reason for this is, on the one hand, the heavy CNC unit, which responds sluggishly to the CNC control signal. On the other hand, the accuracy of the traveled contour depends on the dimensional stability as well as on the geometric and positional tolerances of the glass plate being worked, and on a high synchronicity between the continuous feed-through movement of the flat glass and the synchronized movement of the contouring tool. Consequently, starting point offsets of several tenths of a millimeter can ensue, which result in clearly visible, optical or cosmetic drawbacks, such as visible and tactile transitions of the corners to the lengthwise edges.